Construction for axial piston pump or motor



March 22, 1966 i D ET AL 3,241,495

CONSTRUCTION FOR AXIAL PISTON PUMP R MOTOR Filed Aug. 12, 1963 2Sheets-Sheet 1 55 2l- 1 52 f 44 I /1/ W51 2a 26 1 I 6?? a; f 65 2a 36 L45) 50 r 46 77 March 22, 1966 DIEDRlCH ET AL 3,241,495

CONSTRUCTION FOR AXIAL PISTON PUMP OR MOTOR Filed Aug. 12, 1965 2Sheets-Sheet 2 {7 95 34 75 gi -f 5/7,; 93 4150 -4eg? g, 47/ 3 e 97 94414a i M 76 Wig Q BY 5%??? United States Patent Office 3,241,495Patented Mar. .22, 1966 3,241,495 CONSTRUCTION FUR AXHAL PISTON PUMP RMUTOR PaulE. Diedrich, Columbus, Darby B. Neir, Worthington,

and Allen E. Tucker, Columbus, Ohio, assignors to American Brake ShoeCompany, New York, N.Y., a

corporation of Delaware Filed Aug. 12, 1963, Ser. No. 301,461 Claims.(Cl. 103-162) This invention relates to an improved construction forfluid pumps or motors of the axial piston type.

Fluid pressure energy translating devices of the axial piston type arecharacterized by a rotatable cylinder barrel which contains a pluralityof reciprocable pistons. The pistons are received in chambers formed inthe cylinder, and have shoes which are universally connected to theirouter ends. The shoes engage a cam plate or swash plate which is tiltedabout an axis transverse to the axis of rotation of the cylinder barrel,so that as the barrel is rotated the pistons reciprocate in theirrespective chambers. Means are provided to hold the shoes in contactwith the cam plate as the cylinder barrel revolves. The barrel isconnected to a shaft for rotation. Ports formed inthe cylinder barrelconnect the piston chambers and the end face of thebarrel, and theseports communicate sequentially with inlet and outlet ports formed in afixed valve surface or port plate in the casing.

This invention is directed to a new construction for fluid pressureenergy translating devices of the axial piston type, which will displaypumping efficiency generally commensurate with that of priorconstructions, but which. is simpler and more economical to manufacture.

The invention is predicated in part upon the determination that a highdegree of operating efficiency can be obtained in an axial piston typepump or motor if rotation or drive power is transferred between theoperating shaft and the cylinder barrel through a coupling or splineconnection which is adjacent the port end surface of the cylinderbarrel, and if one end only of the shaft is journalled in the housing ata position remote from the valve surface and the cylinder barrel isjournalled in the body or casing of the pump in a bearing .which is inline with or generally symmetrical in the plane of the tilting forcesacting on the pistons.

The invention is also predicated in part upon improved means for holdingthe cylinder barrel in sealing engagement with the fixed valve orbearing surface of the casing, and for holding the piston shoes in closeengagement with the swash plate.

In a related aspect, the invention is also directed to means forpositively limiting the extent to which the piston shoes can move awayfrom or off of the swash plate.

The construction which we have invented is applicable to axial pistontype pumps or motors wherein the drive shaft is at either the port endof the casing or at the swash plateend of the casing.

A preferred axial piston type pressure energy translating deviceincorporating the principles of this invention includes a hollow casingdefining an internal chamber at one end of which a planar portingandbearing surface is presented. Fluid ports communicate through the casingwith the porting and bearing surface. An operating shaft extendsinto thechamber in the direction perpendicular to the porting and bearingsurface, and is journalled in the casing at one end only, at a positionremote from the porting and bearing surface. A cylinder barrel having aplurality of axially reciprocable pistons is disposed in the casing, andthe barrel has an end surface which engages the porting and bearingsurface of the casing. The shaft extends axially to the barrel and issplined to the barrel for rotary movement at a position adjacent the endthereof which engages the fixed porting and bearing surface. Pistonshoes are universally connected to the ends of the pistons in thebarrel, and a cam plate is angulated with respect to the axis of thebarrel about an axis transverse to the barrel axis. A shoe hold-downplate engages the shoes. The barrel has an internal bore about the shaftat the end opposite to the porting and bearing surface, and a plunger isslidably received in this bore. The plunger has a spherical head portionwhich is socketed in a cooperating spherical recess in the center of thehold-down plate. A coil spring is compressed in the bore of the cylinderbarrel between the barrel and the inner end of the plunger, such thatthe plunger is urged toward the hold-down plate. The barrel has acylindrical flange portion or skirt which extends around the plunger,and this flange portion is journalled in the casing in a bearing whichis disposed symmetrically about the midpoint of the hold-down plate.

For purposes of illustration, the invention is described hereinprimarily in relation to a device which is to be operated as a pump, butit will be understood that the invention is equally applicable to axialpiston type devices which are to be operated as motors.

FIGURE 1 is a horizontal cross-section through an axial piston pumpincluding the-features of this invention, wherein the drive shaftextends from the cam plate end of the pump;

FIGURE 2 is a vertical cross-section through the pump of FIGURE 1;

FIGURE 3 is an enlarged cross-sectional view of a portion of the pumpshown in FIGURES l and 2; and

FIGURE 4 is a vertical cross-section through a modified embodiment of apump in accordance with this invention, wherein the drive shaft extendsfrom the port end of the pump.

The axial piston pump shown in FIGURES l and 2 has an outer casing 10comprising a hollow, generally cylindrical body 11 and an end cap 12.The end cap 12 is secured at one end of the body 11 by suitable meanssuch as the bolts 13, 13 shown in FIGURE 2. The pump casing 10 presentsan internal chamber 14 within which the operating mechanism is disposed.

The inner face of the cap 12 is provided with a raised annular boss 15which presents a fiat porting and bearing surface 16. A plurality ofwear pads 17 are also formed on the inner face of cap 12, and these arefinished so that their faces are co-planar with the porting and bearingsurface 16.

As illustrated in FIGURE 1, the cap 12 is provided with two ports 18 and19, one of which constitutes the fluid inlet while the other constitutesthe fluid outlet. These ports 18 and 19 communicate at their inner endswith passageways or ports 20 and 21 respectively, which extend throughthe porting and bearing surface 16. A drain port 22 in the cap 12communicates with the chamber 14.

A central opening 26 is formed in body 11 at the end thereofwhich isopposite to cap 12. The operating shaft 27 of the pump extends throughthis opening 26 into chamber 14toward cap 12. The outer face of anantifriction ball bearing assembly 28 is push-fitted in opening 26 andis seated against a-shoulder 29 formed therein. A lock nut 31 isthreaded on shaft 27 at 32 and. bears against a lock washer 33, whichengages the inner race of bearing 28, thereby preventing axial movementof shaft 27 in the direction toward cap 12.

An oil seal 35 is secured around shaft 27 at the outer end of opening26, and is held in constant engagement with shaft 27 by a coil or garterspring 36 to prevent the escape of fluid along the shaft.

The inner end of shaft 37 is splined as at 38. The shaft is coupledthraugh the splines 38 to a cylinder barrel 40. The barrel 40 has acentral bore 41 splined only at the inner end of the bore 41, i.e. onlyat the end adja cent to the cap 12, as indicated at 42 in FIGURE 3. Thisspline connection between the shaft 27 and barrel 40 is relativelyloose, and functions in the nature of a universal joint; that is, itsfunction is not to precisely align or position the cylinder barrel withrespect to the fixed porting and bearing surface 16 and the passagewaysor ports 20 and 21, but rather is solely to transmit power to thebarrel, which is precisely positioned by other means to be described.

The inner end face of the cylinder barrel 40 is provided with a flatporting and bearing surface 44 which is facially engaged with theporting and bearing surface 16 of the end cap 12 and the wear pads 17.Ports 45 extend from piston chambers 46 formed in the cylinder barrel tothis porting and bearing surface 44. As the cylinder barrel revolves,these ports 45 therein alternately register with the ports 20 and 21 inthe cap 12 to establish communication between the chambers 46 in thebarrel and the inlet and outlet ports 18 and 19 in the cap. The severalpiston chambers 46 are disposed in parallel circular relation around theaxis of the cylinder barrel, and contain reciprocable piston elements50, the outer ends of which are formed with generally spherical heads51.

The axial bore 41 in cylinder barrel 40 has an enlarged portion 52outwardly of the splined portion 42. In this enlarged portion 52 isreceived the cylindrical shank 54 (best seen in FIGURE 3) of a plunger55 the outer end 56 of which is generally spherical. The shaft 27 passesthrough an axial bore in the plunger 55. The plunger may be splined tothe shaft in order to prevent relative rotation between them, and forconvenience in mounting the plunger on the shaft. It is pointed out,however, that no rotation is transmitted from the shaft to the barrel 40through the plunger .55, the outer surface of the plunger shank 54 beingslip-titted in the bore 52 of the barrel.

The shaft 27 is provided with an annular groove or neck 58 the diameterof which is smaller than the diameter of the spline 38, and this groove58 extends in both axial directions from the inner end 57 of the plunger55. A split washer 59 is loosely seated or engaged in this groove 58,and is held therein by the bore 52. The axial dimension of split washer59 is less than the axial dimension of groove 58, so that the washer canslide axially in the groove. As will be explained, this washer 59 limitsmovement of the plunger 55 in the direction toward the end cap 12.

A coil spring 60 is disposed in the enlarged part 52 of bore 41,encircling shaft 27, and is compressed between the cylinder barrel andthe split washer 59, whereby the latter is urged toward the plunger 55and whereby the cylinder barrel is urged toward the end cap 12. Thefunction of this spring 60, as will be described, is to urge the pistonshoes against the swash plate of the pump, and to urge the barrelporting and bearing surface 44 against the surface 16.

Piston shoes 62 are connected with the spherical heads 51 of the severalpistons 50 in a ball and socket type connection which provides universalmovement between these elements whereby the bearing surfaces 63 of theshoes can constantly engage the planar bearing surface 64 of a cam plateor swash plate 65. The swash plate 65 is mounted for rotation about anaxis transverse to the axis of shaft 27, on trunnions 67, 67 (shown inFIGURE 1), which are secured in the body 11. The swash plate 65 has anopen center area 68, through which the shaft 27 passes in the embodimentshown in FIGURES 1, 2 and 3. The bearing surface 64 and the swash plate65 can be inclined with respect to the axis of rotation of the cylinderbarrel 40 so that as the latter member revolves, the pistons 50 will bereciprocated in the barrel to draw in and expel fluid through the ports45.

To maintain the shoes 62 in sliding engagement with cam plate surface64-, the shoes are formed with shoulders 70 (FIGURE 3) for engagement bya retaining disk or shoe hold-down plate 72. The hold-down plate 72 hasopenings 73 through which portions of the shoes 62 project forconnection with the piston heads 51. The openings 73 are larger indiameter than the portions of the shoes which pass through them, topermit relative lateral movemena between these elements, as caused bythe angular relation of the axes about which the pistons 50 and theplate 72 revolve.

In its central portion the plate 72 is provided with a sphericallyshaped socket or opening 74 in which the spherical end 56 of the plungeris seated. The spring urges the plunger 55 into engagement with thesocket 74 in the hold-down plate, and thereby urges the holddown platetoward the swash plate 65, normally insuring constant engagement of theshoes 62 with the bearing surface 64. Movement of the shoes off thesurface 62 is limited by the point at which the inner face of the splitWasher 59 (i.e., the left face in the drawings) abuts the left face ofgroove 58 in shaft 27; preferably this movement is no more than about.001.

A particular bearing construction has been provided to support thecylinder barrel. As previously mentioned, the spline connection at theinner end of the cylinder barrel functions in the nature of a universaldrive c0nnection. A radial bearing surface for the cylinder barrel isprovided by a skirt-like flange 77 which projects from the outer end ofthe cylinder barrel, around the plunger 55. The flange 77 is preferablypinned to the barrel as at 80. The outer surface of this flange isaccurately finished to cylindrical contour and serves as the inner racefor a roller bearing assembly 78, the outer race of which ispress-fiitted into a cylindrical surface 79 formed on the interior ofthe body 11. This bearing 78 supports the cylinder barrel for rotationin the body 11.

In the operation of the device as a pump, fluid is drawn into thechambers 46 in the cylinder barrel 40 from one of the ports 18 or 19,and as the barrel is ro-= tated the angulation of the swash plate forcesthe pistons 50 sequentially to the left in the barrel, thereby reducingthe volume of the chambers 46, so that the fluid is expelled underpressure from the chambers to the other port 19 or 18. The angulation orinclination of the swash plate determines the stroke of the pistons, andtherefore determines the fluid displacement in each revolution of thebarrel. As the barrel is rotated, the left end of spring 60 bearsagainst the shoulder at the end of enlarged bore portion 52, and therebyurges the cylinder barrel into engagement with the porting and bearingsurface 16 on the end cap. At its right end spring 60 engages splitWasher 59, which in turn bears against the plunger 55, thereby urgingthe hold-down plate 72 toward the swash plate 65, so that the shoes 62are normally held in constant engagement with the bearing surface 64during the suction portion of the piston stroke. The right face ofgroove 58 is so positioned that the split washer does not bear againstit.

Tests have demonstrated this construction to be very efficient inoperation, yet the construction is such that the pump or motor can bemanufactured economically in comparison to other pumps of similaroperating performance.

In assembling the pump, with the plunger head 56 engaged in its socket'74 in the hold-down plate 72, the lock nut 31 is drawn up tightly, soas to pull the shaft 27 to the right and thereby bring the left face ofgroove 58 into abutment with the split washer 59 and force the shoes 62tightly against the swash plate bearing surface 64. The lock nut 31 isthen loosened sufficiently to permit the shaft 27 to move backpreferably about .001" from the tight initial position; in operation,this clearance will be reduced as pump temperature increases. Thisclearance positively limits the extent to which the shoes can move offthebearing surface 64, since the split washer factor, they are notnecessary for successful operation of the device. Axial movement ofshaft 27 may alternatively be limited, for example, by the provision ofany means which secure the shaft to the bearing 28 and the bearing 28 tothe pump housing.

The pump shown in FIGURE 4 of the drawings is similar to that shown inFIGURES l, 2, and 3, except that the drive shaft, rather than passingthrough the center of the swash plate, passes through the end plate ofthe pump.

With reference to FIGURE 4, the body 11 ofthe pump is secured to an endcap 90 which has an elongated portion 91 having a central bore 92through which the opbearing surface 16. The outer race of this hearing95 is press fitted against a shoulder 96 in bore portion 94,

.and the inner race of the bearing 95 is fitted on shaft 93. Springclips 97 and 98 are seated in shaft grooves on each side of the innerrace of bearing 95, and a spring clip 99 holds the outer race of bearing95 against shoulder 96, so that the shaft 93 is fixed axially withrespect to the pump casing. An oil seal 105, which may be of the typeshown in FIGURES 1 and 2, encircles the shaft 93 at the end of boreportion 94 and prevents fluid from escaping along the shaft.

The inner end of the cap 90 is provided with a raised annular boss orporting surface 16, around which are formed co-planar wear pads 17,similar to the corresponding elements previously described in relationto FIG- URES 1-3. The cap is provided with two ports, only one of which106 is shown in FIGURE 4, and these ports communicate with portscorresponding to ports 20 and 21 shown in FIGURE 1. The cylinder barrel40, pistons 50, shoes 62, hold-down plate 72, cylinder barrel bearing78, and swash plate 65 of this pump are all identical to thosepreviously described.

The drive shaft 93 is splined to the cylinder barrel through the barrelsplines 42, adjacent the cap porting surface 16. The drive shaft 93 isalso preferably splined to the plunger 55 to prevent relative rotationbetween them.

In place of the split washer motion-limiting means described inconnection with the embodiment of FIGURES 1-3, the constructionillustrated in FIGURE 4 utilizes a T-bar motion limiting device betweenthe shaft 93 and plunger 55.

The end of shaft 93 is slotted radially, the inner end of the slot beingshown at 109 in FIGURE 4. A T-shaped bar 110 is received loosely in thisslot. The bar 110 has a head section 111 the dimension of which isgreater than the inside diameter of plunger shank 54. The shank 114 ofT-bar 110 is threaded and carries a nut 115. The nut 115 bears against awasher 112 which abuts the end of shaft 93, thereby limiting movement ofthe T-bar 110 in the direction toward the end cap 90.

The cylindrical shank 54 of the plunger is radially slotted as at 117toward its head 56, and the head sec tion 111 of the T-bar is slidablyengaged in this slot 117. The right side of the T-bar head section 111is positioned, by adjusting nut 115, to limit the movement of theplunger 55 to the left, preferably to about .001" or less. Thisadjustment is achieved by initially tightening nut 115 to hold the shoes62 tightly against surface 64, and then backing off the nut to thedesired adjustment. By the provision of these means, movement of theshoes off the swash plate is positively limited to a small amount. Thespring 60 urges the plunger againstits socket 74, and holds the cylinderbarrel against the head bearing surface 16.

It will be seen that in both embodiments the operating shaft isjournalled at its outer end in the housing of the pump, at a positionremote from the porting and bearing .surface 16, and that the cylinderbarrel is journalled for rotation in a bearing which is disposedsymmetrically about a plane passing through the center of the thrustforces applied to the ball heads 51 of the pistons 50. This arrangementpreserves the thrust balancing features which are described by Ellis H.Born in his US. Patent No. 2,608,159. It will also be apparent that inboth instances the drive power is taken off the shaft through a couplingadjacent the porting and bearing surface, substantially closer to thebearing surface than the journal for the outer end of the shaft, butthat this coupling does no function in the nature of a journal for theshaft or barrel. It is preferred that the center of the spherical end 56of the plunger 55 lie in a plane passing through the centers of thepiston balls 51, and that the pivot axis of the hanger 65, which axispasses through the centers of the trunnions 67 (through the rollerbearing 78), passes through the center of the end 56 of the plunger at apoint which lies on the axis of the shaft 27 or 93 in said plane. Inaddition to the foregoing, it is also highly desirable that thementioned pivot axis of the hanger pass substantially through the centerof the cylinder barrel bearing 78.

With the arrangement above described, the shaft 27 or 93 is supported bytwo spaced bearings, one at 28 or and the other being the bearing 78which supports the cylinder barrel 4t and the shaft 27 or 93 through theplunger 55.

Having described our invention, what is: claimed is:

1. A fluid pressure energy translating device comprising, a hollowcasing defining an internal chamber, means defining a planar bearingsurface at one end of said chamber, fluid ports communicating with saidbearing surface through said casing, an operating shaft extending intosaid chamber normally to said surface and having one end only journalledin said casing at a position spaced from said surface, a cylinder barrelhaving a plurality of reciprocable pistons received therein, said barrelhaving an end surface engaging said bearing surface of said casing, saidshaft extending axially through said barrel, drive coupling means fortransferring rotary movement directly between said shaft and saidbarrel, said coupling means being located at the end of said barrelwhich is adjacent to said bearing surface, piston shoes universallyconnected to the ends of said pistons, a cam plate angulated withrespect to the axis of said barrel about an axis transverse thereto, ashoe hold-down plate engaging said shoes and holding said shoesconstantly in engagement with said cam plate, a plunger received axiallyin said barrel, said plunger having a spherical head portion which iscentrally socketed in said hold-down plate, said plunger being slidableaxially with respect to said barrel, there being no drive connectionbetween said plunger and said barrel, stressed spring means between saidbarrel and said plunger urging the latter toward said hold-down plate,said barrel having a flange portion extending around said plunger andpistons, and bearing means journalling said flange portion in saidcasing, said bearing means being disposed symmetrically about themidpoint of said hold-down plate.

2. In a fluid pressure energy translating device including a casing, anoperating shaft, a cylinder barrel containing pistons, shoes on saidpistons, a cam plate, a holddown plate engaging said shoes, and a valvesurface engaged by an end surface of said barrel; means journalling saidshaft in said casing only at a single bearing remote from said valvesurface, bearing means journalling said barrel in said casing, saidbearing means being aligned axially with the mid-point of said hold-downplate, and a loose spline connection between said shaft and said barrelat the end surface of said barrel which is adjacent said valve surface.

3. In an axial piston type fluid pressure energy translating deviceincluding a casing, a rotatable cylinder barrel in said casing, aninclined cam plate at one end of said cylinder barrel, a plurality ofpistons disposed for movement in said cylinder barrel, a bearing shoeuniversally connected to each piston and having a surface in engagementwith said cam plate, hold-down means for holding said shoe surfaces inengagement with said cam plate, a fixed valve surface, one end of saidcylinder barrel abutting said valve surface, and an operating shaft, theimprovement comprising, a spline directly interconnecting said shaft andsaid cylinder barrel at the end of said barrel which abuts said valvesurface, there being no other rotational interconnection between saidshaft and said barrel, means journalling said shaft in said casing onlyat a position therealong which is remote from said valve surface, asleeve-like plunger slidably received on said shaft, said plunger havinga spherical end portion, a bore in said barrel in which said plunger isslidably received, a spherical socket formed in the center of saidhold-down means, the spherical end portion of said plunger being seatedin the socket of said hold-down means, spring means in said bore betweensaid barrel and said plunger urging the latter into engagement with saidhold-down means, stop means limiting movement of said plunger along saidshaft in the direction toward said valve surface, and bearing meansjournalling said barrel in said casing, said bearing means beingdisposed in alignment with and symmetrically about the mid-point of saidholddown means.

4. In a fluid pressure energy translating device, a casing, a rotatablecylinder barrel in said casing, a cam plate at one end of said cylinderbarrel, a plurality of pistons disposed for movement in said cylinderbarrel, a bearing shoe universally connected to each piston and having asurface in engagement with said cam plate, hold-down means for holdingsaid shoe surfaces in engagement with said cam plate, a fixed valvesurface, one end of said cylinder barrel abutting said valve surface, anoperating shaft, a spline interconnecting said shaft and said cylinderbarrel at the end of said barrel which abuts said valve surface, therebeing no other rotational interconnection between said shaft and saidbarrel, means journalling said shaft in said casing only at the endthereof which is opposite to said valve surface, adjustment means foradjusting the axial position of said shaft with respect to said casing,a plunger slidably received on said shaft, said plunger having a roundedend, a bore in said barrel in which plunger is slidably received, therounded end of said plunger being engaged with said hold-down means,spring means in said bore between said barrel and said plunger urgingthe latter into engagement with said hold-down means, a groove in saidshaft axially aligned therein with the inner end of said plunger in saidbore, a washer slidable axially in said groove and engageable with theinner end of said plunger, said groove limiting movement of said washerand plunger along said shaft in the direction toward said valve surface,and bearing means journalling said barrel in said casing, said bearingmeans being disposed in alignment with and symmetrically about themidpoint of said hold-down means.

5. In a fluid pressure energy translating device, a casing, a rotatablecylinder barrel in said casing, a cam plate at one end of said cylinderbarrel, a plurality of pistons disposed for movement in said cylinderbarrel, a bearing shoe universally connected to each piston and having asurface in engagement with said cam plate, holddown means for holdingsaid shoe surfaces in engagement with said cam plate, a fixed valvesurface, one end of said cylinder barrel abutting said valve surface, anoperating shaft, a spline interconecting said shaft and said cylinderbarrel at the end of said barrel which abuts said valve surface, therebeing no other rotational interconnection between said shaft and saidbarrel, means journalling said shaft in said casing at the end thereofwhich is adjacent to said valve surface, said journalling means beingspaced axially outwardly of said valve surface, a plunger slidablyreceived on said shaft, said plunger having a rounded end, a bore insaid barrel in which plunger is slidably received, the rounded end ofsaid plunger being facially engaged with said hold-down means, springmeans in said bore between said barrel and said plunger urging thelatter into engagement with said hold-down means, a slot in the end ofsaid shaft, a T-bar in said slot, a stop on the shank of said T-bar andabutting the end of said shaft, the head of said T-bar limiting axialmovement of said plunger on said shaft in the direction toward saidvalve surface, and bearing means journalling said barrel in said casing,said bearing means being disposed in alignment with and symmetricallyabout the mid-point of said hold-down means.

References Cited by the Examiner UNITED STATES PATENTS 2,525,498 10/1950Naylor et al l03162 2,633,104 3/1953 Lauck et al. 103-162. 2,743,6745/1956 Shaw 103162 2,776,628 1/1957 Keel 103162 3,139,038 6/1964 Stewart103162 DONLEY I. STOCKING, Primary Examiner.

LAURENCE V. EFNER, Examiner.

1. A FLUID PRESSURE ENERGY TRANSLATING DEVICE COMPRISING, A HOLLOWCASING DEFINING AN INTERNAL CHAMBER, MEANS DEFINING A PLANAR BEARINGSURFACE AT ONE END OF SAID CHAMBER, FLUID PORTS COMMUNICATING WITH SAIDBEARING SURFACE THROUGH SAID CASING, AN OPERATING SHAFT EXTENDING INTOSAID CHAMBER NORMALLY TO SAID SURFACE AND HAVING ONE END ONLY JOURNALLEDIN SAID CASING AT A POSITION SPACED FROM SAID SURFACE, A CYLINDER BARRELHAVING A PLURALITY OF RECIPROCABLE PISTONS RECEIVED THEREIN, SAID BARRELHAVING AN END SURFACE ENGAGING SAID BEARING SURFACE OF SAID CASING, SAIDSHAFT EXTENDING AXIALLY THROUGH SAID BARREL, DRIVE COUPLING MEANS FORTRANSFERRING ROTARY MOVEMENT DIRECTLY BETWEEN SAID SHAFT AND SAIDBARREL, SAID COUPLING MEANS BEING LOCATED AT THE END OF SAID BARRELWHICH IS ADJACENT TO SAID BEARING SURFACE, PISTONS, CAM PLATE ANGULATEDWITH RESPECT ENDS OF SAID PISTONS, A CAM PLATE ANGULATED WITH RESPECT TOTHE AXIS OF SAID BARREL ABOUT AN AXIS TRANSVERSE THERETO, A SHOEHOLD-DOWN PLATE ENGAGING SAID SHOES AND HOLDING SAID SHOES CONSTANTLY INENGAGEMENT WITH SAID CAM PLATE, A PLUNGER RECEIVED AXIALLY IN SAIDBARREL, SAID PLUNGER HAVING A SPHERICAL HEAD PORTION WHICH IS CENTRALLYSOCKETED IN SAID HOLD-DOWN PLATE, SAID PLUNGER BEING SLIDABLE AXIALLYWITH RESPECT TO SAID BARREL, THERE BEING NO DRIVE CONNECTION BETWEENSAID PLUNGER AND SAID BARREL, STRESSED SPRISNG MEANS BEING SAID BARRELAND SAID PLUNGER URGING THE LATTER TOWARD SAID HOLD-DOWN PLATE, SAIDBARREL HAVING A FLANGE PORTION EXTENDING AROUND SAID PLUNGER ANDPISTONS, AND BEARING MEANS JOURNALLING SAID FLANGE PORTION IN SAIDCASING, SAID BEARING MEANS BEING DISPOSED SYMMETRICALLY ABOUT THEMIDPOINT OF SAID HOLD-DOWN PLATE.